Spondylolisthesis is the slippage of all or part of one vertebra onto another. The term is derived from the Greek words spondylos and olisthesis. Wiltse, Macnab, and Newman combined their concepts in what remains the most widely accepted classification. Wiltse and Rothman in 1989 suggested a common congenital component in the etiology of dysplastic and isthmic types and added the postsurgical group to the original classification. The precise anatomy of the pars lesion is extremely important in understanding the pathogenesis. The pars interarticularis is the connecting link between pedicles, transverse processes, laminae, and the two articular facets acting as the pivot center. The L5- S1 articulation, being in the coronal plane, is more stable than the sagittal placement of the L4- 5 facet joint. The fifth lumbar vertebra is stabilized by a large L5 transverse process, which supports strong muscular and ligamentous(iliolumbar) attachment. A n increased lumbar lordosis increases the shear stress at the L4- 5 level. Both congenital and isthmic types are often associated with spina bifida of the L5 or S1 segments. There seems to be a definite sex and racial difference, with black women (1.1%) having the lowest prevalence and white men(6.4%) the highest. The increased prevalence is in A laskan natives and young sportsmen, ranging from 11% to 35%. Repetitive flexion, combined flexion- extension and both forcible hyperextension and rotation of the lumbar spine predispose to a pars stress fracture. Most of isthmic type develops during the first year of school, and by age 7 the prevalence is 4%. A further 1.4% of cases occur during adolescence, most between 11and 15 years of age. In patients under 25 years of age with low back pain and isthmic spondylolisthesis, this lesion is most probably the cause of the symptoms(18.9%). In patients older than 40 years, it is seldom the only cause of low back pain (5.2%). The radicular pain is found in 14% of patients with isthmic spondylolisthesis. Degenerative spondylolisthesis results from long standing intersegmental instability.10% of women over 60 years had a 1st or 2nd degree. It occurs 6 times more often at the L4- 5 level and 5 times more often in women, mostly in those older than 40 years. The patient may have back pain (5.6%) with or without leg pain and/or may have intermittent claudication (80%).

In traumatic spondylolisthesis, acute fractures of the pars interarticularis are rare and are always due to fracture of the other part of the posterior elements caused by severe trauma. In pathological spondylolisthesis, the bony strength is insuffient to resist forward motion of the proximal vertebra on the one below. In postsurgical spondylolisthesis, the most common etiology is extensive decompression with sacrifice of the facet joints.